In the fields of robots, working machines, automobiles, etc. using electromagnetic motors, the weight reduction of drive systems has been demanded. However, because the output densities of the electromagnetic motors depend on their weight, only limited weight reduction is available in actuators using the electromagnetic motors. It has been thus desired to develop a small-sized, lightweight actuator capable of providing high output.
Actuators should satisfy the following conditions: moving parts are displaced to desired positions by a driving force; the moving parts are surely returned to start positions in a nonoperative state; sufficiently large output is provided such that moving parts can move even with a large load; etc. Springs are used as pressure-controlling members to bring the moving parts back to the start positions in a nonoperative state. In a case where a spring has large resiliency, a large driving force is needed to move the moving part against a spring force. It is thus desired to provide a spring displaced by a slight force.
Shape memory alloys have particularly attracted much attention as materials for actuators, because they can be strained as much as about 5% (shape recovery strain). The shape memory alloys are materials that can be returned to their original shapes at transformation temperatures or higher after deformed at certain temperatures. When a shape memory alloy having an austenitic phase, a high-temperature phase, is heat-treated with its shape constrained to memorize the shape, deformed in a martensitic phase, a low-temperature phase, and then heated, it returns to its original shape through a reverse transformation mechanism. This phenomenon is utilized for actuators. However, the shape recovery phenomenon by temperature change needs control by heating and cooling, and particularly thermal diffusion by cooling is a rate-determining step, resulting in low response to temperature control.
Ferromagnetic shape memory alloys excellent in a shape memory response speed have recently attracted much attention as novel materials for actuators. The ferromagnetic shape memory alloys have a phase transition structure (a twin crystal structure). When a magnetic field is applied to the ferromagnetic shape memory alloys, the martensitic unit cells (magnetization vectors in the cells) are reoriented along a magnetic field to induce strain. JP 11-269611 A discloses an iron-based, magnetic shape memory material composed of an Fe—Pd or Fe—Pt alloy, which is subjected to martensitic transformation by the application of a magnetic energy to generate magnetic strain. However, the iron-based magnetic shape memory alloys such as the Fe—Pd alloy and the Fe—Pt alloy have low ductility and thus low workability and mechanical strength as well as economic disadvantage because of high material costs. JP 5-311287 A discloses a Cu-based ferromagnetic shape memory alloy obtained by sintering a compacted mixture of Cu—Al alloy powder and Cu—Al—Mn alloy powder. However, because this Cu-based ferromagnetic shape memory alloy is produced by compacting, sintering and working powder materials, it disadvantageously has low workability and mechanical strength. Further, JP 11-509368 A and JP 2001-329347 A disclose magnetically driven actuators formed by Ni—Mn—Ga alloys. However, the Ni—Mn—Ga alloys are disadvantageous in workability, mechanical strength and repeating characteristics.
Ferromagnetic shape memory Ni—Co—Al alloys having excellent workability and shape recovery ratio and capable of being subjected to martensitic transformation have recently been proposed (see, for instance, JP 2002-129273 A). However, JP 2002-129273 A is silent about their mechanical strength.